The present invention relates to vacuum type circuit interrupters in which movable contact electrodes are disposed within a housing which is sealingly evacuated. The contacts are movable between a closed position in conductive engagement, and an open position where the contacts are spaced apart to form an arcing gap between them. The arc formed during interruption conducts the circuit current and will extinguish at a natural current zero of the alternating current wave. The gap between the spaced apart open contacts will quickly recover to the high vacuum state to withstand the ensuing recovery voltage without a reignition of the arc. Thus, the circuit current is effectively interrupted.
It is well known in such vacuum type circuit interrupters that the current interruption capability of the interrupter can be increased by applying an axial magnetic field. The field direction is along the direction of arcing to reduce the arc voltage and to maintain a diffused arc. This will prevent overheating of the contacts which could lead to reignition of the arc. Data has been presented to this end in the article "Interruption Ability of Vacuum Interrupters Subjected to Axial Magnetic Field", Proceedings of the IEE, Volume 119, pages 1754-1758 (1972). Similar improvements achieved with axial magnetic fields have been reported by others. While the desirability of establishing axial magnetic fields is well known, researchers have continued to search for a practical convenient way of generating such an axial magnetic field. Early work dealt with providing coils outside of the interrupter housing, and more recently as in U.S. Pat. No. 3,244,843, coils have been attached to the rear surface of the contact electrodes. Others, as in U.S. Pat. No. 3,158,722, have attempted to configure the electrode supporting conductive rod in a field generating configuration. More recent attempts are seen in U.S. Pat. Nos. 3,823,287 and 3,852,555.
In U.S. Pat. No. 3,946,179 an axial magnetic field generating means is shown as part of the contact. The field coil is formed by a plurality of radial spokes and connected circumferential rim pieces. This contact will have an axial field generated during normally closed load current carrying operation. The design has limitations on interrupting under high voltage, high current conditions.
The concept of using circumferentially directed magnetic fields to drive the arc around or about the contact surface preventing arc melting of the contact is also well known in the vacuum interrupter contact art. Recent designs in this area include cup-shaped designs with slotted side walls, and a radially inwardly extending lip portion at the contact surface as seen in U.S. Pat. No. 3,836,740. In copending application Ser. No. 540,206, entitled "Cup-Shaped Contacts for Vacuum Interrupters Having a Continuous Annular Contact Surface," which application is owned by the assignee of the present invention, a solid annular contact is provided over the slotted lip portion of a cup-shaped contact.
It is desired that the vacuum interrupter be usable at the highest possible voltage and current ratings. The closed contacts of the interrupter must be designed to carry normal high current load currents with minimum power dissipation, and yet to be able to be separated when a fault is detected at a random point on the power wave. The interrupter must effectively interrupt after being opened at any instantaneous parting current which is many times the normal instantaneous load current. In order to interrupt on high voltage lines which have high parting currents, the contact must first survive the high power constricted arc which is capable of melting the contact and destroying the interrupter structure. The interrupter must also, once having survived the gross melting arc, be able to recover to withstand the high transient recovery voltage impressed across the contacts. As has been pointed out, the normal practice to avoid melting of the arcs is to utilize a circumferentially directed magnetic field force to rotate the arc or to utilize spiraled contacts to move the arc across the contact surfaces. The use of axial magnetic fields has been largely directed to the aspect of being able to maintain a low arc dissipation into the contacts thus enhancing the ability of the interrupter to withstand the high transient recovery voltage and to maintain the extinguishment of the arc. An axial magnetic field provides a low arc voltage and permits a very diffuse arc condition. The high voltage withstand characteristic of an open interrupter is of course dependent upon the distance of contact separation.
The above-described arrangements for producing axial magnetic fields in interrupters all have serious limitations. The normal load current flows continuously through the field inducing coils of some of the prior art devices which leads to a continuous and undesirable power dissipation. More importantly, the axial magnetic field is only effective if no gross melting arcing takes place initially. If the interrupter is to be used in high voltage, high fault current circuits, the parting of the contacts could occur at high fault current values and an axial magnetic field would not control the intense arc formed.
It is the object of the present invention to provide a contact including means for controlling this initial arcing condition and to further provide axial magnetic field means and a contact surface which will prevent arc reignition.